The semiconductor industry is characterized by a trend toward fabricating larger and more complex functions on a given semiconductor chip. The larger and more complex functions are achieved by reducing device sizes and spacing, and by reducing the junction depth of regions formed in the semiconductor substrate. Among the feature sizes which are reduced in size are the width and spacing of interconnecting metal lines, and the contact openings through which the metal lines make electrical contact to active device regions in the substrate. As these feature sizes are reduced, new problems arise which must be solved in order to economically and reliably produce semiconductor devices.
As the junction depth of doped regions in the substrate below the contact opening decreases, steps must be taken to avoid junction spiking by metal layers deposited into the contact opening. To reduce junction spiking, a conductive material, such as titanium, is deposited into the contact then a layer of titanium nitride is formed to overlay the titanium layer. The titanium nitride acts as a diffusion barrier to prevent components of refractory metals and aluminum metal leads from entering the substrate and spiking the junction. The titanium layer functions to adhere the titanium layer to the contact structure, and to reduce native oxides on the substrate surface thereby lowering the electrical resistance in the contact. While the titanium/titanium nitride diffusion barrier has proven successful in preventing junction spiking, a high degree of step coverage must be obtained as the aspect ratio of contact openings increases.
The aspect ratio of a contact opening is defined as the depth of the opening divided by the width of the opening. As the aspect ratio of contact openings increases new device metallization processes are required to achieve adequate bottom surface coverage in a high aspect ratio contact. However, a titanium film with adequate bottom surface coverage cannot be obtained by conventional vapor deposition techniques. Accordingly, collimation sputtering has been developed to improve the conformality of deposited metal films.
Collimated metal layers can be deposited which meet the requirements of film deposition into high aspect ratio contact openings. Although collimation sputtering is an effective deposition method for improved bottom surface coverage in a metallized contact, it has been observed that metal films formed by collimation sputtering possess a high degree of surface roughness. Furthermore, as the collimator in the sputtering system ages, the crystallographic orientation of metal films slowly changes with the age of the collimator. Also, the surface roughness of the deposited films increases as the collimator ages.
An x-ray diffraction pattern taken from a titanium film formed by conventional collimation sputtering is shown in FIG. 1. The x-ray diffraction data indicates that a mixed crystallographic orientation is obtained by the conventional deposition method. The mixture of crystalline orientations increases surface roughness of the titanium film and results in a poor quality titanium/titanium nitride interface in a contact structure. The poor quality interface and increased contact resistance can compromise the diffusion barrier characteristics of the titanium nitride film. Accordingly, new processing methods are necessary for the fabrication of collimated metal layers in high performance contact structures.